Data networking technology has witnessed tremendous growth over the past decade, and is ready to take off in an unprecedented manner as newer services and applications become available to the home and business user. However, most of this development and growth has been in the backhaul networks where high capacity routers and ultra-high capacity optical links have created a truly broadband infrastructure. The so-called last mile—the access link connecting the user to the backhaul infrastructure—remains a bottleneck in terms of the bandwidth and service quality it affords the end-user. It is possible to provide a high quality, high bandwidth access medium by taking “fiber to the home.” However, such a solution is inordinately expensive from the service provider's perspective. Alternate solutions such as ADSL (Asymmetric Digital Subscriber Line) and DOCSIS (Data Over Cable System Interface Specification) based cable access, which make use of the existing access infrastructure, are asymmetric in the bandwidth they provide in the upstream and downstream directions.
The typical communication network deployed today by cable television service providers uses hybrid fiber/coax (HFC) technology. An example of such a network is shown in FIG. 1. The network includes a headend 10 connected to an optical network unit (ONU) 12 over optical fiber cable 11 using analog transmission, trunk amplifiers 14, taps 16, line extenders 18 and coax cable 20 (feeder 22, distribution 24 and drop 26 connected to homes 28). The network is considered hybrid because the connection between the ONU and the headend uses optical fiber cable in a physical star or point-to-point configuration while the connections between the ONU and the homes use coax cable in a tree and branch topology.
An HFC network with a single ONU typically serves anywhere from 500 to 2000 homes. The feeder portion 22 includes trunk amplifiers 14 that are spaced every 2000 to 3000 feet. In the distribution portion 24, taps 16 are added as needed to serve homes passed by the distribution coax cable 20. A tap typically serves between 2 and 8 homes to connect to individual homes over drops 26 that are up to 400 feet in length. Line extenders 18 are added in the distribution to boost the signals as needed.
The tree and branch feeder/distribution portions 22 24 were designed originally for downstream broadcast signal distribution. For example, each of the trunk amplifiers, line extenders and taps was designed for handling downstream signals. Nevertheless, today's networks have been adapted to provide upstream signal transmission also. FIG. 2 shows the typical frequency spectrum for upstream and downstream transmission in the network. Downstream transmission of analog signals from the ONU 12 to the homes 28 generally occupies a bandwidth range that starts at 55 MHz and ends at 550, 750 or 860 MHz, depending on the type of network equipment used. The downstream analog bandwidth is divided into 6 MHz channels (8 MHz in Europe). The upstream transmission from the homes 28 to the ONU 12 is usually specified to occupy the bandwidth range between 5 and 45 MHz. The DOCSIS protocol has been developed for handling bidirectional signal transmission. Newer systems also use a band of frequencies located above the analog downstream band to provide downstream digital services. These digital services are delivered in 6 MHz channels at a typical data rate of 25 Mbps.
There are several problems that can occur with the upstream signal transmission, namely, ingress noise, variable transmission loss and frequency dispersion. Ingress noise in the upstream direction is a problem that is due primarily to poor and irregular grounding of the drop coax cable terminated at the home. Because of the tree and branch topology, homes at the far end of the network experience much greater loss than do the homes that are near to the headend/ONU. In addition, the impulse response can be very different from home to home due to reflections. The variable loss and variable impulse response requires the use of complex signal equalization at the receiver located at the headend/ONU. This equalization can require on the order of milliseconds to converge and can only correct for flat loss in the cable plant. To deal with the frequency dispersion, the DOCSIS protocol may divide the upstream signal into subchannels, such as 10 or 20 subchannels of 1 MHz bandwidth each and uses Quadrature Phase-Shift Keying (QPSK) or 16 QAM (Quadrature Amplitude Modulation) signal modulation. Each such subchannel operates at about 1.5 Mbps for a total upstream bandwidth on the order of 10 to 20 Mbps. Since the upstream bandwidth is shared typically by about 500 homes, a DOCSIS modem at the home typically is restricted to a maximum upstream data rate of 100 Kbps.
As cable service providers modernize their network plant, they have begun to lay optical fiber from the headend to the trunk amplifiers 14. Their intent has been to separate the DOCSIS based digital data onto an optical fiber that is separate from that which carries the analog video signals. In addition, Cable Modem Termination System (CMTS) functionality has been introduced at the trunk amplifiers to allow the customers on each distribution segment to access the DOCSIS bandwidth. However, this approach still suffers from limited bandwidth in both the upstream and downstream directions. Furthermore, this approach continues to suffer from performance problems caused by ingress and the quality of service and delay problems caused by a contention based access scheme in the upstream direction. In addition, due to the shared medium nature of the cable plant, privacy and security are very big concerns in the DOCSIS specification. In such systems it is possible for a customer on the shared medium to receive and decipher unencrypted data from other customers.
Maintaining optimal performance of the network elements can be a challenge in today's networks. For example, trunk amplifiers and line extenders can drift which then requires manual measurements and realignments in the field. Component failure in the feeding branches of the existing system can render an entire neighborhood out of service. In addition, service provisioning requires manual labor in the field. Current network repair is primarily done in a reactionary mode, prompted by phone calls from affected customers.